ASoC: Don't use wm8994->control_data in wm8994_readable_register()

The field is no longer initialised so this will crash if running on
wm8958.
Reported-by: Thomas Abraham <thomas.abraham@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: stable@kernel.org

sound/soc/codecs/wm8994.c

@@ -56,7 +56,7 @@ static int wm8994_retune_mobile_base[] = {
 static int wm8994_readable(struct snd_soc_codec *codec, unsigned int reg)
 {
 	struct wm8994_priv *wm8994 = snd_soc_codec_get_drvdata(codec);
-	struct wm8994 *control = wm8994->control_data;
+	struct wm8994 *control = codec->control_data;
 
 	switch (reg) {
 	case WM8994_GPIO_1: